This invention relates to a load detecting apparatus for an elevator for detecting a difference between load of an elevator cage and a counterweight by measuring the shaft torque of a hoisting device.
In an elevator system, a load torque acting on a hoisting electric motor varies in accordance with the number of passengers in an elevator cage. The variation of the load torque causes a sudden high speed or low speed when the elevator cage is started and decelerated, resulting in the reduced comfort of the passengers riding in the cage as well as requiring more time for the cage to attain its set speed. In order to deal with such problems, there is provided a measuring device comprising elastic members and a microswitch under an elevator cage platform. When passengers get into the cage, the elastic members are bent and the bending of the elastic members is detected by a microswitch for determining a load value of the cage. A hoisting electric motor is controlled in accordance with the determined load value so that the cage is smoothly accelerated and decelerated.
FIGS. 1 and 2 illustrate one example of an elevator system using a conventional load detecting apparatus which is set forth in Japanese Utility Model Laid Open No. 60-2138. The elevator system comprises a hoistway 1, a machine room 2 provided above the hoistway 1 having a floor 2a, through openings 2b provided in the floor 2a, I-shaped support beams 3 disposed on the floor 2a in the corresponding relationship with respect to the side walls of the hoistway 1, machine base 4 supported respectively at both ends thereof by the support beams 3, a base member 5 disposed on the machine bases 4 through anti-vibration rubber 6, a traction sheave 7 whose rotary shaft is disposed in the cross direction with respect to the base member 5 and disposed substantially on the center portion of the base member 5, a deflector sheave 8 supported by a deflector beam 9 which is fixed below the base member 5 and disposed in the corresponding relationship with respect to the traction sheave 7, a bracket 10 secured to the base member 5 for supporting one end of the rotary shaft of the traction sheave 7, a reduction gear composed principally of parallel shaft gears and secured to the base member 5 for controlling the traction sheave 7, an electric motor 12 for driving the reduction gear 11, a cinder concrete 13 disposed on the floor 2a, a hoisting rope 14 wound over the traction sheave 7 and the deflector sheave 8 and having both ends thereof suspended into the hoistway 1 through the openings 2b, a counterweight 16 fixed to one end of the hoisting rope 14, and an elevator cage 15 fixed to the other end of the hoisting rope 14, comprising a cage frame 15a, a cab 15b, elastic members 15c disposed between the cage frame 15a and the cab 15b for supporting the cage platform 15d, a microswitch 15e for detecting the amount of the bending of the elastic members 15c when passengers get into the cage 15, the detected amount being transmitted to the machine room 2 through a cable (not shown), and a counterweight 16.
In the elevator system as stated above, when the load of the cage 15 is detected by means of the microswitch 15e, the difference between the load of the cage and the counterweight (hereinafter referred to as unbalanced load) is calculated. According to the calculated unbalanced load, the proper torque to be applied to the traction sheave 7 and also the electric motor 12 is determined, and the elevator cage 15 is operated accordingly.
As has been described above, the conventional load detecting apparatus for an elevator is composed of a plurality of elastic members and a microswitch. However, when the passengers crowd into one side of the cage and just above one of the elastic members, the amount of bending of the elastic member may be excessively detected when compared with the real load. Namely, if the passengers were standing evenly on the cage platform, the microswitch would not be activated. Furthermore, when the passengers crowd on one side of the cage platform which is relatively far from the microswitch, the amount of bending of the elastic member is detected as being less than the real load by the microswitch. In other words, the load of the cage may be erroneously detected. Besides, a relatively long cable is required from the microswitch to the control board, which may eventually result in a broken cable so that it becomes useless.